1. Field of the Invention
This invention relates to the manipulation of digital data in an electronic document or image, so as to allow such document or image to be securely communicated.
2. Discussion of the Related Art
In the past, data encryption methods were utilized to preserve privacy when data is transferred through a public environment. For example, digital encryption has been used to store data in electromagnetic storage media and for data communication, in order to protect privacy when such media is publicly accessible, or when such data communication occurs in a public network or through the mail system. As another example, the Global Positioning Satellite utilizes digital encryption techniques to encrypt its data prior to broadcasting, so as to restrict access and to protect data integrity. In data encryption, privacy is achieved by encoding clear text into ciphered text. Without proper decryption, it is extremely difficult to construct the original clear text from the ciphered text.
In general, encryption requires specific communication structures and protocols which are different from the corresponding structures and protocols used in a non-encrypted environment. For this reason, the cost of providing data encryption is high. For example, ordinary communication equipment not equipped to execute a decryption algorithm cannot receive ciphered text to reproduce the clear text. If the transmitted data is an image, until it is decrypted, even the encrypted image cannot be viewed, i.e. the image can neither be printed on a hard copy nor be displayed on a video display.
Various methods have been devised to encrypt clear text into ciphered text to preserve data secrecy in data processing and material handling applications. Examples of such methods can be found in the following classes under the current classification scheme of the U.S. Patent Office: (a) Class 283-73, Cryptogram for printed matter; (b) Class 283-17, Cryptography, viewable but with hidden meaning; and (c) class 380-54, Cryptography combined with overlay image. Patents relating to encryption can be found dating back prior to 1950's.
U.S. Pat. No. 3,718,349, entitled "Cipher Device and Method" to Albert W. Small, issued Feb. 27, 1973, discloses a cipher device and a method which utilizes a sheet having obverse and reverse surfaces. The method disclosed in the '349 patent requires matching several physical sheets of image to recover the original data. Such a ciphering process is slow, and provides only limited ciphering capability. Such method is clearly not practical for use in a data communication network.
U.S. Pat. No. 4,529,870, entitled "Cryptographic Identification, Financial Transaction and Credential Device" to David Chaum et. al., issued Jul. 16, 1985, discloses a means for processing a credit card or an I.D. card using personal keys. The device disclosed in the '870 patent is a device for coding, which cannot be used to encrypt a large document or a graphical image.
U.S. Pat. No. 4,972,475, entitled "Authenticating Pseudo-Random Code and Apparatus" to Carl Sant'Anselmo, issued Nov. 20, 1990, discloses a non-linear feedback method for encrypting two-dimensional information. The method in the '475 patent is also an encoding procedure which extends a conventional one-dimensional bar code encryption scheme into a two-dimensional coding scheme, thereby reducing the physical space requirement of the encoded data. However, such a coding process cannot be used to encrypt a large document or a graphical image.
U.S. Pat. No. 4,896,355, entitled "Patterning device for Security Facsimile Systems" to Jerry Iggulden et. al., issued Jan. 23, 1990, discloses a device which divides a document transmission into several separate partial document transmissions to maintain data privacy. However, under the method of the '355 patent, data can be compromised easily, because an unauthorized individual can easily obtain a copy of each separate partial document and reassemble the complete image. Such a method is not suitable for serious applications.
An article entitled "Facsimile with Encrypted Hard Copy" by R. Myers et al, in IBM Technical Disclosure Bulletin Vol. 20 No. 11B, pp. 4994-4995, discloses the concept of using an encrypted hard copy output from a facsimile machine. Myers et al do not provide technical information as to how such a machine capable of decrypting from an encrypted hard copy can be constructed. Myers et al discuss various problems with decryption under such approach, but provide no definite solutions. Myers et al suggest that: (i) use of error correction/detection would be necessary, and (ii) the encrypted message would have to be printed at a lower resolution, with concurrent application of a data compression algorithm.
U.S. Pat. No. 5,159,630, entitled "Facsimile Message Encryption System" to Tseng et al, issued Oct. 27, 1992, filed May 29, 1991, discloses starting and boundary markers to demarcate on a page of facsimile transmission an encrypted area, while all other areas on the page are to be transmitted as clear text. This approach restricts the amount of useful space in a facsimile transmission, and requires the user to use a facsimile transmission in an unfamiliar way, which is contrary to the common practice of sending a cover page designating recipient and routing information, to be followed by transmission of the substantive content. Under the approach of Tseng et al, the substantive content is restricted to the encryption area demarcated by the starting and boundary markers. The methods of the '630 patent are hence cumbersome to apply.
Finally, a policy issue being considered by law enforcement agencies today relates to the use of encryption by criminals to communicate with each other in furtherance of criminal activities. Law enforcement agencies would prefer to acquire decryption keys to all commercially available encryption devices, so as to allow such law enforcement agencies to decrypt any encrypted message under appropriate circumstances and procedures (e.g. under a court order). For example, there has been significant debate surrounding the encryption standard of the "clipper" integrated circuit. The public and equipment manufacturers in the private sector are vociferously resisting the requirement that governmental agencies be given "master keys" to commercial encryption equipment. It is therefore desirable that a method for protecting data privacy be devised according to which government agencies can, with only moderate difficulty, recover clear text from a protected message, while at the same time providing adequate data privacy for most commercial applications.